In recent years, brushless DC motors have been increasingly installed in ventilating air blowers, such as a ventilation fan, a range hood fan, and an air cleaner because brushless DC motors offer great efficiency, lower power consumption, and excellence in durability. In such brushless DC motors, when an abnormality such as locking or overloading occurs, the temperature of a winding abnormally rises, and accordingly, insulation breakdown or poor insulation occurs in the winding, and, at the worst, there is a possibility that the motors ignite. Therefore, brushless DC motors have the function of, when an abnormality occurs, detecting the abnormality and substantially preventing a rise in the temperature of a winding or other components.
As an example of such type of conventional brushless DC motors, a brushless DC motor illustrated in FIG. 9 and FIG. 10 has been known.
Hereinafter, the configuration of the brushless DC motor will be described with reference to FIG. 9 and FIG. 10.
FIG. 9 is a block diagram illustrating a function of a conventional brushless DC motor. As illustrated in FIG. 9, windings U, V, and W of brushless DC motor 101 are connected to a plurality of switching elements 102 configured to transmit driving signals Vu, Vv, and Vw to windings U, V, and D, respectively. Each of switching elements 102 is provided with transistors Q1, Q2, Q3, Q4, Q5, and Q6. Each of transistors Q1 to Q6 is connected in parallel to a diode. Driving signal Vu for winding U is outputted from a connection point between transistor Q1 and transistor Q4; driving signal Vv for winding V is outputted from a connection point between transistor Q2 and transistor Q5; and driving signal Vw for winding W is outputted from a connection point between transistor Q3 and transistor Q6.
Collectors of transistors Q1 to Q3 are connected to an anode of DC power supply 103. Emitters of transistors Q4 to Q6 are connected to a cathode of DC power supply 103.
Position detector 104 is connected to switching element ON/OFF signal generating unit 105. Switching element ON/OFF signal generating unit 105 is connected to driving unit 106 configured to output a control signal for turning on/off each of transistors Q1 to Q6 of switching element 102. A signal outputted from switching element ON/OFF signal generating unit 105 is inputted into driving unit 106.
Furthermore, switching element ON/OFF signal generating unit 105 is connected to speed instruction unit 107 and overcurrent detector 108.
Using a comparator, speed instruction unit 107 compares number-of-revolution command signal 109 for determining the number of revolutions of brushless DC motor 101 with a triangular wave from a triangular wave generating circuit. Then, speed instruction unit 107 outputs, to switching element ON/OFF signal generating unit 105, a duty for an on-time of transistors Q1 to Q6 according to a predetermined rotation speed, thereby driving brushless DC motor 101. A motor current flows through current detection resistor 110, the motor current having the same magnitude as the magnitude of current flowing through windings U, V, and W of brushless DC motor 101 via switching elements 102.
FIG. 10 is a block diagram of overcurrent detector 108.
Using comparator 112, overcurrent detector 108 compares a voltage generated when a motor current having flown through each of windings U, V, and W flows through current detection resistor 110 with reference voltage 111. When the voltage generated in current detection resistor 110 is larger than Vref, that is, reference voltage 111, comparator 112 outputs a signal for turning off transistors Q1 to Q6 to switching element ON/OFF signal generating unit 105. When receiving the signal inputted from comparator 112, switching element ON/OFF signal generating unit 105 turns off transistors Q1 to Q6 via driving unit 106.
Temperature-sensitive resistance element 113 whose a resistance value increases in response to a temperature rise, common power supply 114 for a drive circuit, and partial-pressure resistance 115 having a resistance value R1 sufficiently larger than that of the temperature-sensitive resistance element at a normal temperature are connected between current detection resistor 110 and comparator 112. Then, voltage V0 to be inputted to comparator 112 is obtained by adding, to a voltage generated in current limiting resistance 110, voltage V0=E*Rt/(Rt+R1), where E is a voltage of common power supply 114, and Rt is a resistance value of temperature-sensitive resistance element 113 at a certain temperature. That is, by increasing the number of signals for turning off transistors Q1 to Q6, switching element ON/OFF signal generating unit 105 reduces current flowing through brushless DC motor 101, thereby substantially preventing a rise in the temperatures of windings U, V, and W.